1. Field of the Invention
The present invention relates to radar devices which transmit radar waves and receive arrival radar waves (or reflected radar waves) which are reflected by objects, and detect an object on the basis of the arrival radar waves. The present invention further relates to calibration systems and methods of calibrating whether or not a current environment is, in which a radar device is installed, suitable for the calibration of a radar device.
2. Description of the Related Art
There have been known radar devices having a transmitting antenna and a receiving antenna. The transmitting antenna transmits radar waves toward a front area in front of a driver's vehicle (or an own vehicle) equipped with a radar device. The receiving antenna is an array antenna composed of a plurality of antenna elements arranged in an array arrangement. The receiving antenna receives arrival radar waves (or reflected radar waves) which are reflected by objects. The radar device mounted to the own vehicle detects a distance between the radar device and an object which reflects radar waves. The radar device generates an azimuth (or an arrival azimuth) of the object on the basis of the transmitted radar waves and the received arrival radar waves. The radar device generates object information including the distance and the arrival azimuth of the arrival radar waves reflected by the object on the basis of the detected distance and the arrival azimuth of the arrival radar waves. For example, a patent document 1, Japanese patent laid open publication No. JP 2008-145178, disclosed such a conventional radar device.
Such a radar device uses a conventional method of estimating the arrival azimuth of arrival radar waves which are reflected by an object. The conventional method generates a correlation matrix which indicates a correlation between signals of the arrival radar waves received by the antenna elements of the receiving antenna. The method executes eigenvalue-decomposition of the generated correlation matrix in order to estimate the number of the arrival radar waves. The method detects the arrival azimuth of the estimated arrival radar waves on the basis of angle spectrum. There have been known methods such as MUSIC (Multiple Signal Classification) and ESPRIT (Estimation of signal Parameters via Rotational Invariance Technique) which detect such an arrival azimuth of the estimated arrival radar waves.
By the way, when the radar device is mounted to the own vehicle, it is necessary to harmonize an arrangement reference axis of the receiving antenna of the radar device with a predetermined mounting reference axis of the own vehicle. This is because the estimated arrival azimuth of radar waves in the radar device is represented by using a coordinate system on the basis of a direction of the receiving antenna. If the relationship between the coordinate system of the receiving antenna and the coordinate system of the own vehicle is not known, it is difficult to estimate the correct arrival azimuth of arrival radar waves with high accuracy.
In order to solve the above problem, a known method of calibrating the radar device mounted to a motor vehicle executes a step of detecting a correspondence between the coordinate system of the receiving antenna and the coordinate system of the motor vehicle. In a regular calibration step capable of calibrating the axis such as the mounting reference axis of the radar device and the arrangement reference axis of the receiving antenna, an calibration reflector (which is capable of reflecting radar waves) is installed at a predetermined position in an inspection environment, and a calibration system then transmits radar waves to the reflector. The radar device estimates an arrival azimuth of radar waves reflected by the reflector. Finally, the calibration system detects whether or not the arrangement reference axis of the receiving antenna of the radar device is correctly aligned with the mounting reference axis of the motor vehicle on the basis of the estimated arrival azimuth.
However, the conventional calibration system usually executes the calibration in a manufacturing factory of motor vehicles or an auto repair maintenance factory. In general, there are many objects (or various calibration obstacles) in addition to the reflector in those inspection environments, which reflect radar waves in an irradiation range of the radar device. These objects cause incorrect detection when using the calibration system.
The presence of such various calibration obstacles prevents the radar device from detecting a correct location of the reflector. In addition, the presence of these calibration obstacles causes a difficulty for the calibration system to detect whether or not the arrangement reference axis of the receiving antenna is correctly aligned with the mounting reference axis of a motor vehicle equipped with a radar device.
That is, it is necessary for the calibration system to detect whether or not the current environment is, in which the radar device is installed, suitable for the correct calibration before the calibration system detects whether or not the arrangement reference axis of the receiving antenna of the radar device mounted to the motor vehicle is correctly aligned with the mounting reference axis of the motor vehicle.